Plastic containers having flexible bottom walls to accommodate the development of a vacuum therewithin are well-known and commercially available, a particularly desirable container being that which is disclosed in Blanchard U.S. Pat. No. 3,409,167. While such containers are entirely satisfactory under most circumstances, when they are made in small sizes and/or with relatively rigid resins, the functioning of the bottom wall may not be entirely adequate for certain applications. Thus, it is believed that, with the more rigid resins and the smaller end wall diameters, flexure of the bottom wall is inhibited somewhat, thereby limiting the degree of vacuum compensation which can be obtained. Thinning of the bottom wall in an effort to increase its flexibility may be disadvantageous in diminishing resistance to oxygen and water-vapor permeation therethrough, and also in increasing the tendency of the bottom wall to crease or pucker under vacuum.
The need to compensate for pressure differentials arises most dramatically in the case of containers which are filled with hot products, and thereafter promptly sealed. Upon cooling, thermal contractions, lowering of vapor pressure, and possible chemical reaction cause a partial vacuum to form, and this is particularly true when there is appreciable headspace above the product which, as a practical matter, will almost always be the case. This, in turn, may produce creasing and/or buckling of the container, if measures are not taken to reduce the vacuum level. These problems are, of course, particularly acute when a thermoplastic container is filled with a hot product, because of the diminished rigidity which the resin will typically exhibit at the elevated temperatures employed.
In addition to the foregoing, it is highly desirable to utilize, as a closure for such plastic container bodies, a metal end of the sort which is normally used with a metal can body. As is well known, such ends and bodies are generally joined by a double seaming technique, whereby flanges on the two members are interfolded so as to effect secure interengagement therebetween. The formation of such a double seam requires the application of a considerable compressive axial force, or top load, upon the closure. This, in turn, requires the body to possess a significant level of crush resistance, again with the difficulties attendant thereto being exacerbated when a heated thermoplastic body is involved. Finally, the application of such axial force will tend to deform a flexible bottom, thereby mechanically reducing the internal volume, causing a corresponding reduction in vacuum-compensating capacity.
It goes without saying that many of the foregoing difficulties can be avoided, or at least alleviated, by producing the plastic body with wall gauges which are sufficiently heavy to resist the forces involved. This, of course, is an unattractive alternative, not only because of the excessive cost for materials that would be involved, but also because of the disadvantages (in terms of shipping and the like) which the increased weight would engender.
Plastic containers of the present sort offer significant advantages in food packaging applications. Included in the variety of products which may be so packaged are gelled foodstuffs, such as fruit jellies. However, it has been found that, upon cooling of a jelly introduced into a rigid, unyielding, high-barrier, plastic-bodied container, a significant number of bubbles are formed and frozen within the product. One of the advantages of a plastic container is the aesthetic appeal which the use of a transparent resin can offer, and such appeal is seriously diminished by the presence of such bubbles. While it may be thought that the bubbles are formed by permeation of gas through the wall of the container under the influence of the internal vacuum which develops, this theory seems untenable in light of the respective gas permeabilities of the plastic material and the gelled product. More particularly, the permeability of the latter is much higher than that of the former, making it dubious that the vacuum force would be sufficient to draw the gas through the wall of the body, but not through the product, so that bubbles would be trapped within the latter. Accordingly one would not expect the relief of vacuum to dissipate any bubbles which are formed, or to inhibit their formation in the first instance.
Accordingly, it is a primary object of the present invention to provide a novel plastic body which is adapted for use in an hermetically sealed package, and which provides means for the relief of internal vacuum.
It is also an object of the invention to provide such a body, and a package utilizing the same, in relatively small sizes, using a relatively rigid synthetic resinous material having good oxygen and water vapor barrier properties, while minimizing the total amount of material utilized in fabrication, yet without sacrifice of such barrier properties.
Another object of the invention is to provide a body of the foregoing description which is well-suited for hot-filling, and for sealing with a closure that is double seamed thereonto.
Still another object of the invention is to provide a package employing a body of the foregoing type and containing a gelled product which is substantially free of trapped bubbles.